Tunneling magnetoresistive effect element and spin MOS field-effect transistor

ABSTRACT

A magnetoresistive effect element includes a first ferromagnetic layer, Cr layer, Heusler alloy layer, barrier layer, and second ferromagnetic layer. The first ferromagnetic layer has the body-centered cubic lattice structure. The Cr layer is formed on the first ferromagnetic layer and has the body-centered cubic lattice structure. The Heusler alloy layer is formed on the Cr layer. The barrier layer is formed on the Heusler alloy layer. The second ferromagnetic layer is formed on the barrier layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2008-005041, filed Jan. 11, 2008,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the tunneling magnetoresistive effectelement and spin MOS field-effect transistor using a Heusler alloy.

2. Description of the Related Art

Recently, a magnetic memory (magnetic random access memory [MRAM]) usingthe tunneling magnetoresistive effect (TMR) element (or a magnetictunnel junction [MTJ] element) having a sandwiched structure including aferromagnetic material/insulator/ferromagnetic material as a memoryelement has been proposed. This device is used as a memory by fixing (ormaintaining) spins in one ferromagnetic material layer (a fixed layer orreference layer), and controlling (or changing) spins in the otherferromagnetic material layer (a free layer or recording layer), therebychanging the resistance between the two layers in the sandwichedstructure. The resistance decreases when the spins in the fixed layerand free layer are parallel, and increases when they are antiparallel.The magnetoresistive change ratio (TMR ratio) as an index of this spinefficiency is a few 10% at room temperature a few years ago, but hasreached 500% in recent years. This widens the range of possibility ofthe device not only as an MRAM but also as various spin devices. As anexample, a spin MOS field-effect transistor (spin MOSFET) combined withan MTJ element has been proposed. This makes a double resistance changeby the gate electrode and TMR ratio feasible by combining the MTJelement with the spin MOSFET obtained by adding the degree of freedom ofspins to carriers.

It is important to increase the TMR ratio in order to realize ahigh-efficiency magnetic memory or spin MOSFET. To this end, it isnecessary to use a ferromagnetic material having a high spinpolarization ratio (P). When a semi-metallic material in which P=100% isused, the TMR ratio is theoretically infinite from Julliere's law.Candidates of a room-temperature, semi-metallic material are CrO₂,Fe₃O₄, and a Heusler alloy. Recently, Co-based Heusler alloys haveachieved high TMR ratios, so spin devices using these alloys areexpected. A Heusler alloy (also called a full-Heusler alloy) is ageneral term for intermetallic compounds having a chemical compositionrepresented by X₂YZ where X is a Co-, Ni-, or Cu-based transition metalelement or noble metal element in the periodic table, Y is an Mn-, V-,or Ti-based transition metal, and Z is a main group element of groupsIII to V. The Heusler alloy X₂YZ can be classified into three types ofcrystal structures in accordance with the regularities of X, Y, and Z.The L2₁ structure is a structure having the highest regularity, in whichX≠Y≠Z, i.e., the three elements can be distinguished from each other.The B2 structure is a structure having the second highest regularity, inwhich X≠Y=Z. The A2 structure is a structure in which X=Y=Z, i.e., thethree elements cannot be distinguished from each other.

To achieve a high TMR ratio by using the Heusler alloy, epitaxial growthof a regular crystal structure is indispensable when forming a stackedstructure. The past research reveals that an epitaxially grown B2structure or L2₁ structure is necessary to achieve a high spinpolarization ratio of the Heusler alloy. Especially when using theHeusler alloy in a spin MOSFET, a technique of forming a highly regularHeusler alloy on a semiconductor is indispensable (e.g., N. Tezuka, et.al., Appl. Phys. Lett. 89(2006)112514).

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda magnetoresistive effect element comprising: a first ferromagneticlayer having a body-centered cubic lattice structure; a Cr layer formedon the first ferromagnetic layer and having the body-centered cubiclattice structure; a Heusler alloy layer formed on the Cr layer; abarrier layer formed on the Heusler alloy layer; and a secondferromagnetic layer formed on the barrier layer.

According to a second aspect of the present invention, there is provideda spin MOS field-effect transistor comprising, in a source and a drain,a structure including: an MgO layer formed on a semiconductor substrate;a first ferromagnetic layer formed on the MgO layer and having abody-centered cubic lattice structure; a Cr layer formed on the firstferromagnetic layer and having the body-centered cubic latticestructure; and a Heusler alloy layer formed on the Cr layer.

According to a third aspect of the present invention, there is provideda spin MOS field-effect transistor comprising, in at least one of asource and a drain, a structure including: an MgO layer formed on asemiconductor substrate; a first ferromagnetic layer formed on the MgOlayer and having a body-centered cubic lattice structure; a Cr layerformed on the first ferromagnetic layer and having the body-centeredcubic lattice structure; a Heusler alloy layer formed on the Cr layer; abarrier layer formed on the Heusler alloy layer; and a secondferromagnetic layer formed on the barrier layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view of a tunneling magnetoresistive effectelement of the first embodiment;

FIG. 2 is a sectional view of a spin MOSFET of the second embodiment;

FIG. 3 is a sectional view of a spin MOSFET of the third embodiment;

FIG. 4 is a sectional view of a memory cell in an MRAM of the fourthembodiment;

FIG. 5 is a sectional view of an MTJ element in the memory cell of thefourth embodiment;

FIG. 6 is a sectional view of another MTJ element in the memory cell ofthe fourth embodiment;

FIG. 7 is a sectional view of still another MTJ element in the memorycell of the fourth embodiment;

FIG. 8 is a sectional view of a TMR head of the fifth embodiment;

FIG. 9 is a sectional view of a tunneling magnetoresistive effectelement as a comparative example;

FIG. 10 is a sectional view of a tunneling magnetoresistive effectelement as an example; and

FIG. 11 is a graph showing the results of X-ray diffraction (XRD)evaluation of the comparative example and example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be explained below withreference to the accompanying drawing. In the following explanation, thesame reference numerals denote the same parts throughout the drawings.

First Embodiment

First, a tunneling magnetoresistive effect element of the firstembodiment of the present invention will be explained below. FIG. 1 is asectional view showing the structure of the tunneling magnetoresistiveeffect element of the first embodiment.

As shown in FIG. 1, the tunneling magnetoresistive effect element of thefirst embodiment has a structure in which an MgO layer 11, aferromagnetic layer 12 having the body-centered cubic (bcc) structure,and a Cr layer 13 having the bcc structure are sequentially formed on asemiconductor single-crystal substrate 10, and a tunnelingmagnetoresistive effect element using a Heusler alloy is formed on theCr layer 13.

More specifically, the MgO layer 11 is formed on the surface of thesemiconductor substrate 10, and the ferromagnetic layer 12 having thebcc structure is formed on the MgO layer 11. In addition, the Cr layer13 having the bcc structure is formed on the ferromagnetic layer 12, anda Heusler alloy layer 14 is formed on the Cr layer 13. Furthermore, atunnel barrier layer (e.g., an MgO layer) 15 is formed on the Heusleralloy layer 14, and a Heusler alloy layer 16 is formed on the tunnelbarrier layer 15. One of the Heusler alloy layers 14 and 16 is regardedas a free layer (magnetization free layer or recording layer), amagnetization thereof is made invertible (or variable), and the other isregarded as a pinned layer (magnetization fixed layer or referencelayer), and a magnetization thereof is fixed (or invariable). Theresistance of the tunneling magnetoresistive effect element can bechanged by changing the relative relationship between the magnetizationin the free layer and that in the pinned layer by, e.g., a spininjection method or current-induced magnetic field application method.Note that the MgO layer 11 is desirably (001)-oriented. Note also thatthe Heusler alloy layer 16 may also be a ferromagnetic layer, e.g.,Fe—Co or Fe—Co—B.

In the tunneling magnetoresistive effect element having the abovestructure, an epitaxially grown Heusler alloy having a highly regularcrystal structure (the B2 structure or L2₁ structure) can be formed onthe semiconductor substrate. In addition, the roughness of the interfacebetween the tunnel barrier layer and Heusler alloy layer in the MTJelement can be reduced by forming the Cr layer having the bcc structurebetween the ferromagnetic layer and Heusler alloy layer. Accordingly, itis possible to form a tunneling magnetoresistive effect element usingthe Heusler alloy and having a high TMR ratio. Also, carriers to whichthe degree of freedom of spins is added can be conducted in thesemiconductor substrate because the ferromagnetic layer having the bccstructure is formed.

The film thickness of the Cr layer having the bcc structure ispreferably 3 nm or less. When the film thickness of the Cr layer is 3 nmor less, the lattice constant of the ferromagnetic layer having the bccstructure can be taken over. This makes it possible to control thelattice constant of the Heusler alloy formed on the Cr layer by theferromagnetic layer having the bcc structure and the Cr layer having thebcc structure as the underlying layers.

Second Embodiment

A spin MOSFET of the second embodiment of the present invention will beexplained below. FIG. 2 is a sectional view showing the structure of thespin MOSFET of the second embodiment.

As shown in FIG. 2, impurity diffusion layers 10A as a source and drainare formed in the surface region of a semiconductor substrate 10 by ionimplantation. MgO layers 11 are formed on the impurity diffusion layers10A. Ferromagnetic layers 12 having the bcc structure are formed on theMgO layers 11. Cr layers 13 having the bcc structure are formed on theferromagnetic layers 12, and epitaxially grown Heusler alloy layers 14are formed on the Cr layers 13. Antiferromagnetic coupling may also beformed between the ferromagnetic layers 12 and Heusler alloy layers 14via the Cr layers 13. Furthermore, a gate insulating film 21 is formedon the semiconductor substrate 10 between the source and drain, and agate electrode 22 is formed on the gate insulating film 21. Note thatthe MgO layers 11 are desirably (001)-oriented.

In the spin MOSFET of this embodiment, one of the Heusler alloy layers14 on the source and drain is regarded as a free layer (magnetizationfree layer or recording layer), a magnetization thereof is madeinvertible (or variable), and the other is regarded as a pinned layer(magnetization fixed layer or reference layer), and a magnetizationthereof is fixed (or invariable). It is only necessary to use a magneticmaterial having a high holding force as the pinned layer, or form anantiferromagnetic layer on the pinned layer. The resistance of thetunneling magnetoresistive effect element can be changed by changing therelative relationship between the magnetization in the free layer andthat in the pinned layer. The magnetization in the free layer can bechanged by means of, e.g., a method of injecting spins via a channel orthe like, or a current-induced magnetic field application method.

A method of manufacturing the spin MOSFET shown in FIG. 2 will beexplained below. First, a channel region is formed in a semiconductorsubstrate 10 by ion implantation and annealing. After that, a siliconoxide film 21 and polysilicon film 22, for example, are sequentiallyformed on the semiconductor substrate 10. Then, as shown in FIG. 2, agate insulating film 21 and gate electrode 22 are formed by etching awaythe silicon oxide film 21 and polysilicon film 22 from prospectivesource and drain portions.

Impurity diffusion layers 10A are formed by ion implantation andannealing in the prospective source and drain portions of the surfaceregion of the semiconductor substrate 10. Subsequently, an MgO layer 11is formed on the impurity diffusion layers 10A by sputtering. Aferromagnetic layer 12 having the bcc structure is formed on the MgOlayer 11 by sputtering. A Cr layer 13 having the bcc structure is formedon the ferromagnetic layer 12 by sputtering. In addition, an epitaxiallygrown Heusler alloy layer 14 is formed on the Cr layer 13. Then, sourceand drain electrodes are formed by patterning the MgO layer 11,ferromagnetic layer 12, Cr layer 13, and Heusler alloy layer 14 by meansof, e.g., liftoff, ion milling, or RIE. In this manner, the spin MOSFETshown in FIG. 2 is manufactured.

Next, the conditions of ion implantation for forming the impuritydiffusion layers 10A in the spin MOSFET will be explained below. As anion candidate to be substituted, it is possible to use, e.g., phosphorus(P), arsenic (As), or boron (B). The projection range is desirably 20 nmor less, and the acceleration voltage is preferably 20 keV or less. Thecarrier concentration in the impurity diffusion layer 10A is 10¹⁸ to10²⁰/cm³. The annealing conditions will now be explained. As theannealing method, normal annealing or rapid thermal annealing (RTA) canbe used. Annealing of the impurity diffusion layers can be performedafter the MgO layer 11 is formed, or after the Heusler alloy layer 14 isformed. Annealing is desirably performed after the formation of the MgOlayer 11 and before the subsequent film formation. When annealing isperformed after the MgO layer 11 is formed, the crystallinity of the MgOlayer 11 can be improved.

In the spin MOSFET having the above structure, an epitaxially grownHeusler alloy having a highly regular crystal structure (the B2structure or L2₁ structure) can be formed on the semiconductorsubstrate. In addition, the roughness of the interface between thetunnel barrier layer and Heusler alloy layer in the MTJ element can bereduced by forming the Cr layer having the bcc structure between theferromagnetic layer and Heusler alloy layer. Also, carriers to which thedegree of freedom of spins is added can be conducted in thesemiconductor substrate because the ferromagnetic layer 12 having thebcc structure is formed. That is, carriers having spins to be suppliedto the source or drain are conducted as they tunnel through the MgOlayer 11 as a barrier layer via the ferromagnetic layer 12 having thebcc structure.

The film thickness of the Cr layer having the bcc structure ispreferably 3 nm or less. When the film thickness of the Cr layer is 3 nmor less, the lattice constant of the ferromagnetic layer having the bccstructure can be taken over. This makes it possible to control thelattice constant of the Heusler alloy formed on the Cr layer by theferromagnetic layer having the bcc structure and the Cr layer having thebcc structure as the underlying layers.

Third Embodiment

A spin MOSFET of the third embodiment of the present invention will beexplained below. FIG. 3 is a sectional view showing the structure of thespin MOSFET of the third embodiment.

As shown in FIG. 3, impurity diffusion layers 10A as a source and drainare formed in the surface region of a semiconductor substrate 10 by ionimplantation and annealing. MgO layers 11 are formed on the impuritydiffusion layers 10A. Ferromagnetic layers 12 having the bcc structureare formed on the MgO layers 11. Cr layers 13 having the bcc structureare formed on the ferromagnetic layers 12, and epitaxially grown Heusleralloy layers 14 are formed on the Cr layers 13. Antiferromagneticcoupling may also be formed between the ferromagnetic layers 12 andHeusler alloy layers 14 via the Cr layers 13. Also, a tunnel barrierlayer 15 is formed in at least one of the source and drain, and aHeusler alloy layer 16 is formed on the tunnel barrier layer 15. Inother words, in at least one of the source and drain, a TMR element (astacked structure including the Heusler alloy layer 14, tunnel barrierlayer 15, and Heusler alloy layer 16) using the Heusler alloy is formedon the Cr layer 13 and ferromagnetic layer 12 on the MgO layer 11. Inaddition, a gate insulating film 21 is formed on the semiconductorsubstrate 10 between the source and drain, and a gate electrode 22 isformed on the gate insulating film 21. Note that the MgO layers 11 aredesirably (001)-oriented. Note also that the Heusler alloy layer 16 mayalso be a ferromagnetic layer, e.g., Fe—Co or Fe—Co—B.

In the spin MOSFET of this embodiment, one of the Heusler alloy layers14 on the source and drain is regarded as a free layer (magnetizationfree layer or recording layer), a magnetization thereof is madeinvertible (or variable), and the other is regarded as a pinned layer(magnetization fixed layer or reference layer), and a magnetizationthereof is fixed (or invariable). Furthermore, the Heusler alloy layer16 also functions as a pinned layer, and the magnetization direction inthe Heusler alloy layer 16 may be the same as or opposite to that in theHeusler alloy layer (pinned layer) 14. It is only necessary to use amagnetic material having a high holding force as these pinned layers, orform an antiferromagnetic layer on each pinned layer. The resistance ofthe tunneling magnetoresistive effect element can be changed by changingthe relative relationship between the magnetization in the free layerand that in the pinned layer. The magnetization in the free layer can bechanged by means of, e.g., a method of injecting spins via a channel orthe like, or a current-induced magnetic field application method.

A method of manufacturing the spin MOSFET shown in FIG. 3 will beexplained below. First, a channel region is formed in a semiconductorsubstrate 10 by ion implantation and annealing. After that, a siliconoxide film 21 and polysilicon film 22, for example, are sequentiallyformed on the semiconductor substrate 10. Then, as shown in FIG. 3, agate insulating film 21 and gate electrode 22 are formed by etching awaythe silicon oxide film 21 and polysilicon film 22 from prospectivesource and drain portions.

Impurity diffusion layers 10A are formed by ion implantation andannealing in the prospective source and drain portions of the surfaceregion of the semiconductor substrate 10. Subsequently, an MgO layer 11is formed on the impurity diffusion layers 10A by sputtering. Aferromagnetic layer 12 having the bcc structure is formed on the MgOlayer 11 by sputtering. A Cr layer 13 having the bcc structure is formedon the ferromagnetic layer 12 by sputtering. In addition, an epitaxiallygrown Heusler alloy layer 14 is formed on the Cr layer 13. Then, sourceand drain electrodes are formed by patterning the MgO layer 11,ferromagnetic layer 12, Cr layer 13, and Heusler alloy layer 14 by meansof, e.g., liftoff, ion milling, or RIE.

After that, a resist film that exposes only one of the source and drainelectrodes and covers the rest is formed. Subsequently, a tunnel barrierlayer 15 and Heusler alloy layer 16 are stacked on the semiconductorsubstrate 10, and the resist film is removed, thereby forming the tunnelbarrier layer 15 and Heusler alloy layer 16 on only one of the sourceand drain electrodes. As a consequence, an MTJ element is formed on atleast one of the source and drain electrodes. In this way, the spinMOSFET shown in FIG. 3 is manufactured.

In the first, second, and third embodiments explained above, as theferromagnetic layer 12 having the bcc structure, it is possible to useiron (Fe), iron cobalt (Fe—Co), iron manganese (Fe—Mn), an alloycontaining at least one of vanadium (V), niobium (Nb), molybdenum (Mo),tantalum (Ta), nickel (Ni), and tungsten (W) in addition to Fe, Fe—Co,or Fe—Mn, or a stacked structure of these ferromagnetic materials. It isalso possible to adjust the lattice constant of the Heusler alloy bythat of the ferromagnetic layer 12 having the bcc structure bydecreasing the film thickness of the Cr layer 13 having the bccstructure.

In addition, the first, second, and third embodiments explained aboveare characterized in that the semiconductor substrate is a substratehaving one of an Si single crystal, Ge single crystal, GaAs singlecrystal, and Si—Ge single crystal on at least the surface, or asilicon-on-insulator (SOI) substrate.

In the spin MOSFET having the above structure, an epitaxially grownHeusler alloy having a highly regular crystal structure (the B2structure or L2₁ structure) can be formed on the semiconductorsubstrate. In addition, the roughness of the interface between thetunnel barrier layer and Heusler alloy layer in the MTJ element can bereduced by forming the Cr layer having the bcc structure between theferromagnetic layer and Heusler alloy layer. Accordingly, it is possibleto form a tunneling magnetoresistive effect element using the Heusleralloy and having a high TMR ratio. Also, carriers to which the degree offreedom of spins is added can be conducted in the semiconductorsubstrate because the ferromagnetic layer having the bcc structure isformed.

The film thickness of the Cr layer having the bcc structure ispreferably 3 nm or less. When the film thickness of the Cr layer is 3 nmor less, the lattice constant of the ferromagnetic layer having the bccstructure can be taken over. This makes it possible to control thelattice constant of the formed Heusler alloy by the ferromagnetic layerhaving the bcc structure and the Cr layer having the bcc structure asthe underlying layers.

Fourth Embodiment

An MRAM of the fourth embodiment of the present invention will beexplained below. An MTJ element having a Heusler alloy is used in amemory cell of this MRAM. FIG. 4 is a sectional view showing thestructure of the memory cell of the MRAM of the fourth embodiment.

As shown in FIG. 4, the memory cell of the MRAM of the fourth embodimenthas a structure in which an electrode layer, polycrystalline metalunderlying interconnection 37, MTJ element 38, and metal via (or metalhard mask) 39 are sequentially formed on a transistor formed on asemiconductor substrate 30, and a bit line 40 is formed on top of thestructure.

The structure of the memory cell of the MRAM will be described in detailbelow. Element isolation regions 31 are formed in the semiconductorsubstrate 30, and source and drain regions 32 are formed in thesemiconductor substrate sandwiched between the element isolation regions31. A gate insulating film 33 is formed on the semiconductor substrate30 between the source and drain regions. A gate electrode 34 is formedon the gate insulating film 33. An interlayer dielectric film 35 isformed on the semiconductor substrate 30. In the interlayer dielectricfilm 35 on the source or drain region 32, first, second, and thirdinterconnections M1, M2, and M3 are sequentially formed with contactplugs 36 inserted between them. The polycrystalline metal underlyinginterconnection 37 is formed on the contact plug 36 on the thirdinterconnection M3. The MTJ element 38 is formed on the polycrystallinemetal underlying interconnection 37. The metal via (or metal hard mask)39 is formed on the MTJ element 38. The bit line 40 is formed on themetal via 39.

FIG. 5 shows details of the sectional structure of the MTJ element 38.As shown in FIG. 5, an MgO layer 11 is formed on the polycrystallinemetal underlying interconnection 37. A ferromagnetic layer 12 having thebcc structure and a Cr layer 13 having the bcc structure aresequentially formed on the MgO layer 11. A Heusler alloy layer 14 (arecording layer), tunnel barrier layer (e.g., an MgO layer) 15, andHeusler alloy layer 16 are sequentially formed on the Cr layer 13. Inaddition, a ferromagnetic layer (e.g., a CoFe layer) 17,antiferromagnetic layer 18, and cap layer 19 are sequentially formed onthe Heusler alloy layer 16 in order to form a top fixed layer. Note thatthe MgO layer 11 is desirably (001)-oriented.

The MTJ element 38 may also have a dual fixed layer structure as shownin FIG. 6. An MgO layer 11, a ferromagnetic layer 12 having the bccstructure, a Cr layer 13 having the bcc structure, a Heusler alloy layer14, a tunnel barrier layer 15, and a Heusler alloy layer 16 (a recordinglayer) are sequentially formed on the polycrystalline metal underlyinginterconnection 37. In addition, a tunnel barrier layer (e.g., an MgOlayer) 23, ferromagnetic layer 17, antiferromagnetic layer 18, and caplayer 19 are sequentially formed on the Heusler alloy layer 16. To allowthe ferromagnetic layer 12 and Heusler alloy layer 14 to function aspinned layers, ferromagnetic layers having a high holding force needonly be used as these layers. Antiferromagnetic coupling may also beformed between the ferromagnetic layer 12 and Heusler alloy layer 14 viathe Cr layer 13. It is also possible to form an antiferromagnetic layerbetween the MgO layer 11 and ferromagnetic layer 12, and use anantiferromagnetic layer instead of the MgO layer 11.

Furthermore, although FIG. 5 shows the case where the MTJ element 38 isformed as a top fixed layer, the MTJ element 38 may also be formed as abottom fixed layer as shown in FIG. 7. That is, it is also possible toform an MTJ structure in which an antiferromagnetic layer 18, aferromagnetic layer 17 having the bcc structure, a Cr layer 13 havingthe bcc structure, an epitaxially grown Heusler alloy layer 14, a tunnelbarrier layer 15, a Heusler alloy layer 16 (a recording layer), and acap layer 19 are sequentially stacked on the polycrystalline metalunderlying interconnection 37. Antiferromagnetic coupling may also beformed between the ferromagnetic layer 17 and Heusler alloy layer 14 viathe Cr layer 13. It is also possible to form an MgO layer between thepolycrystalline metal underlying interconnection 37 andantiferromagnetic layer 18. A material such as Al, Au, Ag, Pt, Cu, or Crcan be used as the polycrystalline metal underlying interconnection 37,and a semiconductor material such as polysilicon may also be used as theunderlying interconnection. Furthermore, the Heusler alloy layer 16shown in FIGS. 5, 6, and 7 may also be a ferromagnetic layer, e.g.,Fe—Co or Fe—Co—B.

In the fourth embodiment, a tunneling magnetoresistive effect elementusing a Heusler alloy having a highly regular crystal structure can beformed. This makes it possible to provide an MRAM including a tunnelingmagnetoresistive effect element having a high TMR ratio.

Fifth Embodiment

A TMR head of the fifth embodiment of the present invention will beexplained below. This TMR head is formed by using an MTJ element, andused in a hard disk drive (HDD). FIG. 8 is a sectional view showing thestructure of the TMR head of the fifth embodiment.

As shown in FIG. 8, the TMR head has a structure in which the MTJelement is sandwiched between a lower electrode layer 41 and upperelectrode layer 42. The MTJ element has a structure in which anamorphous layer (metal amorphous layer or insulator amorphous layer) 20,an MgO layer 11, a ferromagnetic layer 12 having the bcc structure, a Crlayer 13 having the bcc structure, an epitaxially grown Heusler alloylayer 14, a tunnel barrier layer 15, and a Heusler alloy layer 16 aresequentially stacked on the lower electrode layer 41. Antiferromagneticcoupling may also be formed between the ferromagnetic layer 12 andHeusler alloy layer 14 via the Cr layer 13.

More specifically, as shown in FIG. 8, the amorphous layer 20, the MgOlayer 11, the ferromagnetic layer 12, the Cr layer 13, the Heusler alloylayer 14, the tunnel barrier layer (e.g., an MgO layer) 15, the Heusleralloy layer 16, a ferromagnetic layer (e.g., a CoFe layer) 17, anantiferromagnetic layer 18, and a cap layer 19 are sequentially formedon the lower electrode layer (magnetic shield layer) 41. In addition,the upper electrode layer (magnetic shield layer) 42 is formed on thecap layer 19. An insulating film 24 is formed between the lowerelectrode layer 41 and upper electrode layer 42. Note that the MgO layer11 is desirably (001)-oriented. Note also that the Heusler alloy layer16 may also be a ferromagnetic layer, e.g., Fe—Co or Fe—Co—B.

In the fourth and fifth embodiments explained above, as theferromagnetic layer 12 having the bcc structure, it is possible to useiron (Fe), iron cobalt (Fe—Co), iron manganese (Fe—Mn), an alloycontaining at least one of vanadium (V), niobium (Nb), molybdenum (Mo),tantalum (Ta), nickel (Ni), and tungsten (W) in addition to Fe, Fe—Co,or Fe—Mn, or a stacked structure of these ferromagnetic materials. Also,the film thickness of the Cr layer 13 having the bcc structure ispreferably 3 nm or less in order to adjust the lattice constant of theHeusler alloy by that of the ferromagnetic layer 12 having the bccstructure. The film thickness of each of the amorphous layer 20 and MgOlayer 11 on the lower electrode layer 41 is desirably a film thicknessthat allows tunneling of carriers. When the amorphous layer 20 is aconductor, the film thickness of the MgO layer 11 is desirably 3 nm orless by which carriers can tunnel through the layer. When the amorphouslayer 20 is an insulator, the total film thickness of the amorphouslayer 20 and MgO layer 11 is desirably 3 nm or less.

MgO or Al₂O₃ can be used as the insulator layer as the tunnel barrierlayer in the MTJ element. The film thickness of the tunnel barrier layerin the MTJ element is desirably a film thickness that causes no carrierspin relaxation and allows tunneling of carriers, and preferably 3 nm orless that is much smaller than the spin diffusion length.

In the fifth embodiment, a tunneling magnetoresistive effect elementusing a Heusler alloy having a highly regular crystal structure can beformed. This makes it possible to provide a TMR head including atunneling magnetoresistive effect element having a high TMR ratio.

The embodiments of the present invention will be explained in moredetail below by way of its comparative example and example.

As a comparative example, a tunneling magnetoresistive effect elementhaving the Heusler alloy was manufactured. The procedure of themanufacture was as follows. FIG. 9 is a sectional view showing thestructure of the tunneling magnetoresistive effect element of thecomparative example.

First, the surface of a (001)-oriented MgO substrate 11 was cleaned bysputter cleaning. Then, a 20-nm-thick ferromagnetic layer (CoFe layer)43 having the bcc structure was formed on the MgO substrate 11 bysputtering. In addition, a 30-nm-thick Heusler alloy layer 14 made ofCo₂Al_(0.5)Si_(0.5), a 1-nm-thick tunnel barrier layer (MgO layer) 15,and a 5-nm-thick Heusler alloy layer 16 made of Co₂FeAl_(0.5)Si_(0.5)were sequentially formed on the ferromagnetic layer 43 by sputtering.Furthermore, to generate an exchange coupling magnetic field, a10-nm-thick antiferromagnetic layer (IrMn layer) 44 was formed on theHeusler alloy layer 16, and a 7-nm-thick cap layer (Ru layer) 45 wasformed on the antiferromagnetic layer 44. After the film formation, thetunneling magnetoresistive effect element of the comparative examplemanufactured by means of the above procedure was annealed in a vacuum at400° C.

When a current-in-plane technique (CIPT) apparatus was used to measurethe magnetic reluctance of the manufactured tunneling magnetoresistiveeffect element, the TMR ratio was 15.6%, i.e., was very low. The TMRratio was low even when an Fe layer was used instead of the CoFe layer43 in the tunneling magnetoresistive effect element shown in FIG. 9.

As an example of the present invention, therefore, a tunnelingmagnetoresistive effect element shown in FIG. 10 was manufactured. Theprocedure of the manufacture was as follows. FIG. 10 is a sectional viewshowing the structure of the tunneling magnetoresistive effect elementof the example.

The tunneling magnetoresistive effect element of the example shown inFIG. 10 had a structure in which a 1-nm-thick Cr layer 13 was formed inthe interface between the CoFe layer 43 having the bcc structure and theHeusler alloy layer 14 made of Co₂FeAl_(0.5)Si_(0.5) in the structureshown in FIG. 9. The use of the Cr layer 13 makes it possible to reducethe roughness of the interface between the Heusler alloy layer 14 andtunnel barrier layer 15 in the MTJ element. After the film formation,the tunneling magnetoresistive effect element having this structure wasannealed in a vacuum at 400° C.

When a current in-plane technique (CIPT) apparatus was used to measurethe magnetic reluctance of the tunneling magnetoresistive effect elementof the example, the TMR ratio was 116.0%, i.e., was high.

FIG. 11 shows the results of evaluation performed the crystal structuresof the formed Heusler alloys by an X-ray diffraction apparatus. FIG. 11reveals that the formed Heusler alloy had the B2 structure because a(002) peak was observed. FIG. 11 also indicates that even when the1-nm-thick Cr film 13 was formed in the interface between the CoFe layer43 having the bcc structure and the Heusler alloy layer 14 made ofCo₂FeAl_(0.5)Si_(0.5), the Heusler alloy layer 14 maintained the B2structure, and the lattice constant remained unchanged. This shows thatwhen a thin Cr layer is formed in the interface between a ferromagneticlayer having the bcc structure and a Heusler alloy layer, it is possibleto form a Heusler alloy that takes over the lattice constant of theferromagnetic layer having the bcc structure. Note that CFAS in FIG. 11represents the Heusler alloy made of Co₂FeAl_(0.5)Si_(0.5). Note alsothat an expression such as CFAS(30)/Cr(1)/CoFe(20) represents astructure in which a Cr layer (film thickness=1 nm) and a Heusler alloy(film thickness=30 nm) made of Co₂FeAl_(0.5)Si_(0.5) are sequentiallyformed on a CoFe layer (film thickness=20 nm).

Also, a Cr layer having the bcc structure and a CoFe layer having thebcc structure were formed instead of the CoFe layer 43 having the bccstructure in the structure shown in FIG. 10. More specifically, a Crlayer having the bcc structure was formed on an MgO substrate 11, and aCoFe layer having the bcc structure was formed on this Cr layer. A Crlayer 13 was formed on the CoFe layer. The TMR ratio of a tunnelingmagnetoresistive effect element having this structure further increased.Accordingly, the above-mentioned elements of a ferromagnetic layerhaving the bcc structure may also be partially substituted with otherelements as long as the bcc structure is maintained. Even in this case,the crystallinity of the Heusler alloy on the ferromagnetic layerimproves, and as a consequence the magnetic characteristics and the likeof the alloy also improve. The above ferromagnetic layer may also have astacked structure.

From the foregoing, the example of the present invention makes itpossible to form a Heusler alloy having a highly regular crystalstructure, and realize a tunneling magnetoresistive effect elementhaving a high TMR ratio. When the structure shown in FIG. 10 of thisexample is formed on a semiconductor substrate, carriers to which thedegree of freedom of spins is added can be conducted from theferromagnetic layer having the bcc structure into the semiconductorsubstrate.

The embodiments of the present invention can provide a tunnelingmagnetoresistive effect element that uses a Heusler alloy having ahighly regular crystal structure and has a high TMR ratio, and a spinMOS field-effect transistor using this tunneling magnetoresistive effectelement.

Also, the above-mentioned embodiments can be practiced not only singlybut also in the form of an appropriate combination. Furthermore, theaforesaid embodiments include inventions in various stages, soinventions in various stages can also be extracted by appropriatelycombining a plurality of constituent elements disclosed in theembodiments.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A spin MOS field-effect transistor comprising, in a source and adrain, a structure including: an MgO layer formed on a semiconductorsubstrate; a first ferromagnetic layer formed on the MgO layer andhaving a body-centered cubic lattice structure; a Cr layer formed on thefirst ferromagnetic layer and having the body-centered cubic latticestructure, the Cr layer having a film thickness of 3 nm or less; and aHeusler alloy layer formed on the Cr layer and having one of a B2structure and an L2₁ structure.
 2. The transistor according to claim 1,wherein the first ferromagnetic layer contains a material selected fromthe group consisting of iron (Fe), iron cobalt (Fe Co), iron manganese(Fe Mn), alloys containing at least one of vanadium (V), niobium (Nb),molybdenum (Mo), tantalum (Ta), nickel (Ni), and tungsten (W) inaddition to Fe, Fe—Co, and Fe—Mn, and stacked structures of thematerials described above.
 3. The transistor according to claim 1,wherein the semiconductor substrate is made of a material selected fromthe group consisting of Si, Ge, GaAs, and Si—Ge.